Ip – Difference between :: and ::1

ipip addressipv6protocol-theory

I am confused about following netstat output. There are two kind of local ports.
The first is:

[::]8100

The second is:

[::1]8100

I know that [::1] is a loopback address for IPv6. My question is:

What does [::] mean?

Thank you in advance for your help.

Best Answer

:: is the unspecified address (0:0:0:0:0:0:0:0), and it is only used in packets as the source address of a host that does not yet have an address and is trying to get an address assigned. What you see in the output means that a process is binding to port 8100 for all destination addresses in the host.

::1 is the loopback address (0:0:0:0:0:0:0:1), and anything sent to that address will loop back inside the host. What you see means a process is binding to port 8100 on the loopback address, and that means anything sent to the loopback address on that port will go the that process.

Neither address can be seen as a destination address on a network outside of the host, and the unspecified address can only be used as a source address for a host that has not yet been assigned an address but is looking for an address. The loopback address should never be used as a source address on a network outside of the host.

RFC 4219, IP Version 6 Addressing Architecture explains how the two addresses are compressed in section 2.2. Text Representation of Addresses:

  1. Due to some methods of allocating certain styles of IPv6 addresses, it will be common for addresses to contain long strings of zero bits. In order to make writing addresses containing zero bits easier, a special syntax is available to compress the zeros. The use of "::" indicates one or more groups of 16 bits of zeros. The "::" can only appear once in an address. The "::" can also be used to compress leading or trailing zeros in an address.

    For example, the following addresses

         2001:DB8:0:0:8:800:200C:417A   a unicast address
     FF01:0:0:0:0:0:0:101           a multicast address
     0:0:0:0:0:0:0:1                the loopback address
     0:0:0:0:0:0:0:0                the unspecified address

    may be represented as

         2001:DB8::8:800:200C:417A      a unicast address
     FF01::101                      a multicast address
     ::1                            the loopback address
     ::                             the unspecified address

It further goes on to explain both the unspecified and loopback addresses:

2.5.2. The Unspecified Address

The address 0:0:0:0:0:0:0:0 is called the unspecified address. It must never be assigned to any node. It indicates the absence of an address. One example of its use is in the Source Address field of any IPv6 packets sent by an initializing host before it has learned its own address.

The unspecified address must not be used as the destination address of IPv6 packets or in IPv6 Routing headers. An IPv6 packet with a source address of unspecified must never be forwarded by an IPv6 router.

2.5.3. The Loopback Address

The unicast address 0:0:0:0:0:0:0:1 is called the loopback address. It may be used by a node to send an IPv6 packet to itself. It must not be assigned to any physical interface. It is treated as having Link-Local scope, and may be thought of as the Link-Local unicast address of a virtual interface (typically called the "loopback interface") to an imaginary link that goes nowhere.

The loopback address must not be used as the source address in IPv6 packets that are sent outside of a single node. An IPv6 packet with a destination address of loopback must never be sent outside of a single node and must never be forwarded by an IPv6 router. A packet received on an interface with a destination address of loopback must be dropped.

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